А) 71/12 - 13/6 -7/4
1) 71/12 - 13/6 = 71/12 - 26/12= 55/12
2) 55/12 - 7/4 = 55/12 - 21/12 = 34/12 = 17/6 = 2 целых 5/6
б) (15 - 83/20) - ( 12 -53/15). 1)15 - 83/20= 300/20 - 83/20 = 217/20
2) 180/15 - 53/15 = 127/15
3) 217/20 - 127/15 = 651/60 - 508/60= 143/60 = 2 целых 23/60